It is important for the insulated wires used in televisions, electromagnetic cookers, copying machines, computers and other electronic appliances to be safe against fire, and such fire-resistant, insulated wires as authorized by the Electric Appliances Regulations in Japan, the UL Standards in the United States and the CSA (Canadian Standard Association) Standards in Canada are used for the purpose.
In addition, since the fire-resistant, insulated wires used in the high-voltage parts of electronic appliances deal direct current of so high a voltage of 10 to 40 kV, safety against high voltage is also important.
As the method of evaluation of high-voltage wires for electronic appliances, one described in the Subject 758 in the UL Standards are known. In the evaluating method described there, (1) the high-voltage cut-through test and (2) the fire-resistance test are regarded to be technically difficult to pass.
FIG. 2 is given for explaining the high-voltage cut-through test according to the Subject 758 in the UL Standards. As shown in the figure, to both ends of a wire 13 which is hung down over two parallel drill rods 11, 1/32" in diameter, are applied loads 10 of 1 pound, in a bath kept at the temperature rating, and when 1.5 times as high a rated voltage is applied from the source of direct current 12, breakdown must not take place within 7 hours.
FIG. 3 is provided for the explanation of the fire-resistance test according to the UL Standards. As shown in the figure, when a wire 18, placed perpendicular in an enclosure 14 for the protection from the movement of surrounding air, is exposed to fire from a burner 15, the fire must be extinguished within 60 seconds, and an absorbent cotton 17 placed on the bottom must not be set on fire from dropped burning matter and moreover a sheet of kraft paper 16 placed above must not be set on fire nor be scorched.
In order to satisfy these two requirements, double layered wire comprising inner polyethylene insulation and outer flame retarding resin composition jacket have been employed.
For instance, a wire in which polyethylene with melting point higher than 105.degree. C. is coated over a conductor for passing the high-voltage cut through test has been previously disclosed. According to this disclosure, a flame retarding jacket comprising mainly ethylene-vinylacetate-vinylchloride copolymer is coated over the polyethylene insulation. (See Japanese Patent Publication No. 41786/1977)
It has also been reported that a similar double layered wire outer layer which comprises polyvinylchloride grafted chlorinated polyethylene (Japanese Patent Publication No. 15058/1979), and chlorosulfonated polyethylene can be employed. (Japanese Patent Laid-Open Publication No. 42755/1974)
These insulated wires achieve VW-1 level flame retardancy by means of covering polyethlene with a highly fire resisting resin composition, which can compensate easy flamability of polyethylene while keeping the excellent insulation property and tracking resistance of polyethylene.
However, these insulated wires have an upper limit of temperature rating of 105.degree. C., because the outer resin composition layer have a PVC moiety or chlorine-containing monomers as a repeating unit.
There are also some examples of single layered high-voltage lead wires in prior art, the insulation of these wires are flame retardant resin like a chlorinated polyethylene. However, these insulated wires also have upper limit of temperature rating of 105.degree. C., so these high-voltage wires cannot be used at higher temperatures.
A method to make a flame retarded polyethylene composition by adding the flame retardant is also known. However, the withstand voltage characteristics of these flame retarded polyethylene composition is inferior, so the high-voltage lead wires with excellent properties have not been obtained by using such a flame retarded polyethylene composition as an insulation.
On the other hand, requirements for the use of high-voltage lead wires have increasingly been severe. The size of electronic appliances have become increasingly smaller and many functions have been required with the result of increased amount of wiring and increased requirement for higher safety and higher heat resistance in wiring materials. To meet such requirements, employment of some highly heat-resistant resins other than polyolefins may be considerable.
Generally the high-voltage lead wires used in an environment at higher than 150.degree. C. are wires with vulcanized silicon rubber.
Silicon rubber insulated high-voltage lead wires have excellent flexibility and electrical properties, however, the silicon rubber insulation is apt to tear by being scraped with metal edge during wiring operations. This sometimes causes cracks in the insulation and leads to dielectrical break-down trouble in the worst case.
Some techniques, for example, covering the silicon rubber insulated wires with braided glass tubing or resin tubing are known. However, in general silicon rubber insulated wires are expensive, and the total cost becomes much higher when such protecting tubings are adopted. And in addition, the diameter of the protected wire becomes too large and the handling of these insulations in wiring operations becomes difficult.
High-voltage wires for direct current utilizing such fluorocarbon resins as tetrafluoroethylene and tetrafluoroethylene-hexafluoroethylene copolymer as the insulating materials are also known. They are excellent wires with heat resistance at higher than 150.degree. C., but they are more expensive than silicone-rubber insulated wires and accordingly they are not practical for use other than for a specific purpose.
Thus, the first objective of the present invention is to provide a heat-resistant high-voltage lead wire for direct current with temperature rating of 125.degree. C., which can be manufactured efficiently using inexpensive materials.
The second objective of the present invention is to provide a heat-resistant high-voltage lead wire for direct current with extremely large utility value corresponding to the requirement of higher heat resistant property in the fields of using high-voltage wires for direct current.